The use of digital transmission systems requires the ability to reconstruct the transmitted digital signal after it has traveled through a noisy, dispersive medium. Signal reconstruction entails forming samples of the received digital signal at discrete times. The amplitude of each sample indicates whether the amplitude of the digital signal at the sample time is greater or less than some decision threshold.
In certain digital transmission systems, such as coaxial cable, the transmission medium is well-controlled and only slight signal delay and distortion occurs. Accordingly, the received digital signal can be regenerated with an acceptable error rate using fixed sampling times. In other digital transmission systems, however, such as those utilizing radio links, the signal distortion and delay introduced in transmission is uncontrollable and often unpredictable. Signal regeneration in such systems using fixed sampling times results in an unacceptable error rate for telecommunications applications.
It is well-known that regeneration errors can be reduced if the sampling times coincide with the so-called "signal-eyes" of changing dimensions and positions. Such signal-eyes are defined by the ensemble of all signal waveforms over the baud interval. Moreover, regeneration errors can generally be reduced even further by the alignment of the sampling times with a predetermined position within the signal-eyes. This predetermined position is typically at the signal-eye center.
Various techniques have been used to track the signal-eyes and vary the sampling times. For example, in U.S. Pat. No. 3,534,273 to Thomas, issued Oct. 13, 1970, a recursive technique, requiring rather elaborate circuitry, is utilized to continually monitor the signal-eye boundaries. Once the boundaries are determined, the sampling times are adjusted to coincide with the center of the signal-eyes. Another eye tracking technique (See U.S. Pat. No. 3,404,232 to Burford, issued Oct. 1, 1968) compares a sample of the digital signal taken at a primary with samples taken at secondary sampling times. All sampling times occur within the signal-eyes with the secondary sampling times straddling the primary sampling time. If the amplitude of each sample relative to a common decision threshold is not the same, a corrective signal is generated which shifts all the sampling times by a time interval which aligns the primary sampling times with the signal-eye centers. The problem with this technique, however, is that no corrective signal is generated for small shifts in the signal-eye position. As a result, the error rate increases to a level which is unacceptable for many telecommunications applications.